Highly infectious symbiont dominates initial uptake in coral juveniles

The majority of reef-building corals acquire their obligate algal symbionts ( Symbiodinium ) from the environment. However, factors shaping the initial establishment of coral–algal symbioses, including parental effects, local environmental conditions and local availability of symbionts, are not well understood. This study monitored the uptake and maintenance of Symbiodinium in juveniles of two common corals, Acropora tenuis and Acropora millepora , that were reciprocally explanted between sites where adult colonies host different types of Symbiodinium . We found that coral juveniles were rapidly dominated by type D Symbiodinium , even though this type is not found in adult colonies (including the parental colonies) in four out of the five study populations. Furthermore, type D Symbiodinium was found in less than one-third of a wide range of coral species ( n  > 50) sampled at the two main study sites, suggesting that its dominance in the acroporid juveniles is not because it is the most abundant local endosymbiotic type. Moreover, dominance by type D was observed irrespective of the light intensity to which juveniles were exposed in a field study. In summary, despite its relatively low abundance in coral assemblages at the study sites and irrespective of the surrounding light environment, type D Symbiodinium is the main symbiont type initially acquired by juveniles of A. millepora and A. tenuis . We conclude that during early ontogeny in these corals, there are few barriers to the uptake of Symbiodinium types which differ from those found in parental colonies, resulting in dominance by a highly infectious and potentially opportunistic symbiont.     

Protection of Coral Larvae from Thermally Induced Oxidative Stress by Redox Nanoparticles

Coral reefs are one of the most biologically diverse and economically important ecosystems on earth. However, the destruction of coral reefs has been reported worldwide owing to rising seawater temperature associated with global warming. In this study, we investigated the potential of a redox nanoparticle (RNP^O) to scavenge reactive oxygen species (ROS), which are overproduced under heat stress and play a crucial role in causing coral mortality. When reef-building coral (Acropora tenuis) larvae, without algal symbionts, were exposed to thermal stress at 33 °C, RNP^O treatment significantly increased the survival rate. Proteome analysis of coral larvae was performed using nano-liquid chromatography-tandem mass spectrometry for the first time. The results revealed that several proteins related to ROS-induced oxidative stress were specifically identified in A. tenuis larvae without RNP^O treatment, whereas these proteins were absent in RNP^O-treated larvae, which suggested that RNP^O effectively scavenged ROS from A. tenuis larvae. Results from this study indicate that RNP^O treatment can reduce ROS in aposymbiotic coral larvae and would be a promising approach for protecting corals from thermal stress.     

Possible self-fertilization in the brooding coral Acropora (Isopora) brueggemanni

We examined gametogenesis and the periodicity of planula release in the brooding coral Acropora (Isopora) brueggemanni (Brook, 1893) on Akajima Island, Okinawa, Japan. We captured the moment when A. brueggemanni would be in the process of self-fertilization. Whole colonies of this species were cultured separately or together with other colonies in plastic containers. We observed no apparent periodicity of planula release in the collected colonies. A few planulae were released intermittently during the observation period, regardless of whether the colonies were cultured individually or with other colonies. Serial paraffin sections of A. brueggemanni showed follicle-like cells surrounding the oocyte during vitellogenesis. In September and October, some spermaries looked half-broken and some ova were surrounded by sperm or spermaries instead of follicle-like cells. Such ova were heart-shaped and may have been cells at the first cleavage stage. These observations suggest that the migration of spermaries and/or oocytes resulted in the close proximity of oocytes and sperm, which would allow self-fertilization. This possibility, together with the production of planula larvae by isolated colonies, suggests that this brooding coral engages in self-fertilization.     

Natural inducers for coral larval metamorphosis

 Coral gametes from Acropora millepora (Ehrenberg, 1834) and from multi-species spawning slicks provided larvae for use in metamorphosis assays with a selection of naturally occurring inducer chemicals. Four species of crustose coralline algae, one non-coralline crustose alga and two branching coralline algae induced larval metamorphosis. However, one additional species of branching coralline algae did not produce a larval response. Metamorphosis was also observed when larvae were exposed to skeleton from the massive coral Goniastrea retiformis (Lamarck, 1816) and to calcified reef rubble, demonstrating metamorphosis is possible in the absence of encrusting algae. Chemical extracts from these algae and the coral skeleton, obtained using either decalcification or simple methanol extraction procedures, also contained active inducers. These results extend the number of crustose algal species known to induce coral metamorphosis, suggest that some inducers may not necessarily be strongly associated with the calcified algal cell walls, and indicate that inducer sources in reef habitats may be more diverse than previously reported. Accepted: 21 May 1999     

Host-symbiont specificity during onset of symbiosis between the dinoflagellates Symbiodinium spp. and planula larvae of the scleractinian coral Fungia scutaria

Many corals which engage in symbioses with dinoflagellates from the genus Symbiodinium (zooxanthellae) produce offspring which initially lack zooxanthellae. These species must choose their symbionts from numerous genetically distinct strains of zooxanthellae co-occurring in the environment. In most cases, symbiosis onset results in an association between a specific host coral and a specific strain of algal symbiont. This is the first study to examine host-symbiont specificity during symbiosis onset in a larval cnidarian, and the first to examine such events in a scleractinian of any life stage. We infected planula larvae of the solitary Hawaiian scleractinian Fungia scutaria with both homologous zooxanthellae, freshly isolated from F. scutaria adults, and heterologous zooxanthellae, isolated from Montipora verrucosa, Porites compressa, and Pocillopora damicornis, three species of scleractinians which co-occur with F. scutaria. We found that homologous zooxanthellae were better able to establish symbioses with larval hosts than were heterologous isolates, by two separate measures: percent of a larval population infected, and densities of zooxanthellae per larva. We also measured algal densities in larvae over a 4-day period until the onset of settlement and metamorphosis. We found no changes in zooxanthella population densities, regardless of zooxanthella type or the light environment in which they were incubated. Strong infection of host larvae with homologous algae compared to heterologous algae suggests that there is a specificity process which occurs sometime during the early stages of infection between the partners, and which results in the establishment of a specific symbiosis.     

Different algal symbionts explain the vertical distribution of dominant reef corals in the eastern Pacific

Symbiotic reef corals occupy the entire photic zone; however, most species have distinct zonation patterns within the light intensity gradient. It is hypothesized that the presence of specific symbionts adapted to different light regimes may determine the vertical distribution of particular hosts. We have tested this hypothesis by genetic and in situ physiological analyses of the algal populations occupying two dominant eastern Pacific corals, over their vertical distribution in the Gulf of California. Our findings indicate that each coral species hosts a distinct algal taxon adapted to a particular light regime. The differential use of light by specific symbiotic dinoflagellates constitutes an important axis for niche diversification and is sufficient to explain the vertical distribution patterns of these two coral species.     

Repair machinery of symbiotic photosynthesis as the primary target of heat stress for reef-building corals

In a coral-algae symbiotic system, heat-dependent photoinhibition of photosystem II (PSII) leads to coral bleaching. When the reef-building coral Acropora digitifera was exposed to light, a moderate increase of temperature induced coral bleaching through photobleaching of algal pigments, but not through expulsion of symbiotic algae. Monitoring of PSII photoinhibition revealed that heat-dependent photoinhibition was ascribed to inhibition of the repair of photodamaged PSII, and heat susceptibility of the repair machinery varied among coral species. We conclude that the efficiency of the photosynthesis repair machinery determines the bleaching susceptibility of coral species under elevated seawater temperatures.     

Food selection of a corallivorous butterflyfish Chaetodon austriacus in the Red Sea, with particular emphasis on size and fish symbionts of corals

The exquisite butterflyfish Chaetodon austriacus feeds mainly on Acropora , Pocillopora , Montipora and Stylophora in the northern Red Sea. Large colonies of Acropora are preferred over smaller colonies and other coral genera, whereas avoidance is indicated for corals that are occupied by certain coral-associated gobiid fishes of the genus Gobiodon . It is suggested that, apart from coral identity, colony size and potential repellent effects of particular coral symbionts are both important sources of variation in the food selectivity of corallivorous chaetodotids.     

First frozen repository for the Great Barrier Reef coral created

To build new tools for the continued protection and propagation of coral from the Great Barrier Reef (GBR), an international group of coral and cryopreservation scientists known as the Reef Recovery Initiative joined forces during the November 2011 mass-spawning event. The outcome was the creation of the first frozen bank for Australian coral from two important GBR reef-building species, Acropora tenuis and Acropora millepora. Approximately 190 frozen samples each with billions of cells were placed into long-term storage. Sperm cells were successfully cryopreserved, and after thawing, samples were used to fertilize eggs, resulting in functioning larvae. Additionally, developing larvae were dissociated, and these pluripotent cells were cryopreserved and viable after thawing. Now, we are in a unique position to move our work from the laboratory to the reefs to develop collaborative, practical conservation management tools to help secure Australia's coral biodiversity.     

Developmentally regulated localization of endosymbiotic dinoflagellates in different tissue layers of coral larvae

In adult cnidarians, symbiotic dinoflagellate Symbiodinium are usually located in the gastrodermis. However, the onset of this endosymbiotic association and its regulation during larval development are unclear. This study examined the distribution of the Symbiodinium population in tissue layers of planula larvae released from the stony coral Euphyllia glabrescens. Symbiodinium were redistributed from the epidermis to the gastrodermis, at a rate that was fastest during early planulation and then decreased prior to metamorphosis. This process indicates that the endosymbiotic activity of coral tissues is developmentally regulated. During the early larval stage, both the epidermis and gastrodermis contained Symbiodinium; then, as the larvae developed toward metamorphosis, the numbers in the epidermis gradually diminished until they were only found in the gastrodermis. The mechanism of redistribution remains unknown, but may be due to a direct translocation and/or change in the proliferation of symbionts in different tissue layers.     

Restriction Fragment Length Polymorphism Analysis of Large Subunit rDNA of Symbiotic Dinoflagellates from Scleractinian Corals in the Zhubi Coral Reef of the Nansha Islands

Zooxanthellae are very important for the coral reef ecosystem. The diversity of coral hosts Is high in the South China Sea, but the diversity of zooxanthellae has not yet been investigated. We chose the Zhubi Coral Reef of the Nansha Islands as the region to be surveyed in the present study because it represents a typical tropical coral reef of the South China Sea and we investigated zooxanthellae diversity In 10 host scleractinlan coral species using polymerase chain reaction （PCR） of the large subunit rRNA and restriction fragment length polymorphlsm （RFLP） patterns. Poclllopora verrucosa, Acropora pefifera, Acropora mlllepora, Fungla fungltes, Galaxea fasclcularls, and Acropora pruinosa harbor Clade C, Goniastrea aspera harbors Clade D, and Acropora formosa harbors Clades D and C. Therefore, the Clade C is the dominant type in the Zhubi Coral Reef of the Nansha Islands. Furthermore, the results of the present also disprove what has been widely accepted, namely that one coral host harbors only one algal symblont. The coral-algal symbiosis Is flexible, which may be an Important mechanism for surviving coral bleaching. Meanwhile, on the basis of the results of the present study, we think that Symblodlnium Clade D may be more tolerant to stress than Symbiodlnlum Clade C.     

Loss of Functional Photosystem II Reaction Centres in Zooxanthellae of Corals Exposed to Bleaching Conditions: Using Fluorescence Rise Kinetics

Mass coral bleaching is linked to elevated sea surface temperatures, 1-2 degrees C above average, during periods of intense light. These conditions induce the expulsion of zooxanthellae from the coral host in response to photosynthetic damage in the algal symbionts. The mechanism that triggers this release has not been clearly established and to further our knowledge of this process, fluorescence rise kinetics have been studied for the first time. Corals that were exposed to elevated temperature (33 degrees C) and light (280 mumol photons m(-2) s(-1)), showed distinct changes in the fast polyphasic induction of chlorophyll-a fluorescence, indicating biophysical changes in the photochemical processes. The fluorescence rise over the first 2000ms was monitored in three species of corals for up to 8 h, with a PEA fluorometer and an imaging-PAM. Pocillopora damicornis showed the least impact on photosynthetic apparatus, while Acropora nobilis was the most sensitive, with Cyphastrea serailia intermediate between the other two species. A. nobilis showed a remarkable capacity for recovery from bleaching conditions. For all three species, a steady decline in the slope of the initial rise and the height of the J-transient was observed, indicating the loss of functional Photosystem II (PS II) centres under elevated-temperature conditions. A significant loss of PS II centres was confirmed by a decline in photochemical quenching when exposed to bleaching stress. Non-photochemical quenching was identified as a significant mechanism for dissipating excess energy as heat under the bleaching conditions. Photophosphorylation could explain this decline in PS II activity. State transitions, a component of non-photochemical quenching, was a probable cause of the high non-photochemical quenching during bleaching and this mechanism is associated with the phosphorylation-induced dissociation of the light harvesting complexes from the PS II reaction centres. This reversible process may account for the coral recovery, particularly in A. nobilis.     

Survival dynamics of reef coral larvae with special consideration of larval size and the genus Acropora

Temporal dynamics of larval survival were examined in vitro in four broadcast-spawning reef coral species, Acropora hyacinthus, A. japonica, A. solitaryensis, and Goniastrea pectinata. Larval size was treated as an important characteristic that may relate to larval lifespan. Two patterns were observed in larval survival dynamics between the three Acropora species (mean initial larval size; 0.05-0.08 mm(3)) and G. pectinata (0.02 mm(3)), based on the timing of a sharp drop in larval survival rates (ca. > 50% reduction over a 1-2 week period). Consequently, the majority of larvae of the three Acropora species had a lifespan of less than 2-3 weeks, whereas those of G. pectinata were extended a further 2-3 weeks despite the smaller larval size. No significant relationship was detected between the initial larval size and larval lifespan in any of the four reef coral species. These results suggest that (1) larval dispersal of spawning Acropora species may be on a more local scale than that of G. pectinata and most other reef coral species previously reported, and (2) larval size is not a good estimator of larval lifespan in reef coral species.     

Patterns of Gene Expression in a Scleractinian Coral Undergoing Natural Bleaching

Coral bleaching is a major threat to coral reefs worldwide and is predicted to intensify with increasing global temperature. This study represents the first investigation of gene expression in an Indo-Pacific coral species undergoing natural bleaching which involved the loss of algal symbionts. Quantitative real-time polymerase chain reaction experiments were conducted to select and evaluate coral internal control genes (ICGs), and to investigate selected coral genes of interest (GOIs) for changes in gene expression in nine colonies of the scleractinian coral Acropora millepora undergoing bleaching at Magnetic Island, Great Barrier Reef, Australia. Among the six ICGs tested, glyceraldehyde 3-phosphate dehydrogenase and the ribosomal protein genes S7 and L9 exhibited the most constant expression levels between samples from healthy-looking colonies and samples from the same colonies when severely bleached a year later. These ICGs were therefore utilised for normalisation of expression data for seven selected GOIs. Of the seven GOIs, homologues of catalase, C-type lectin and chromoprotein genes were significantly up-regulated as a result of bleaching by factors of 1.81, 1.46 and 1.61 (linear mixed models analysis of variance, P < 0.05), respectively. We present these genes as potential coral bleaching response genes. In contrast, three genes, including one putative ICG, showed highly variable levels of expression between coral colonies. Potential variation in microhabitat, gene function unrelated to the stress response and individualised stress responses may influence such differences between colonies and need to be better understood when designing and interpreting future studies of gene expression in natural coral populations.     

Regulation and control of intracellular algae (= zooxanthellae) in hard corals

To examine algal (= zooxanthellae) regulation and control, and the factors determining algal densities in hard corals, the zooxanthellae mitotic index and release rates were regularly determined in branch tips from a colony of a staghorn coral, Acropora formosa, recovering from a coral ''bleaching'' event (the stress-related dissociation of the coral–algal symbiosis). Mathematical models based upon density-dependent decreases in the algal division frequency and increases in algal release rates during the post-bleaching recovery period accurately predict the observed recovery period (ca. 20 weeks). The models suggest that (i) the colony recovered its algal population from the division of the remaining zooxanthellae, and (ii) the continual loss of zooxanthellae significantly slowed the recovery of the coral. Possible reasons for the ''paradoxical'' loss of healthy zooxanthellae from the bleached coral are discussed in terms of endodermal processes occurring in the recovering coral and the redistribution of newly formed zooxanthellae to aposymbiotic host cells. At a steady-state algal density of 2.1 x 10⁶ zooxanthellae cm^-2 at the end of the recovery period, the zooxanthellae would have to form a double layer of cells in the coral tissues, consistent with microscopic observations. Neighbouring colonies of A. formosa with inherently higher algal densities possess proportionately smaller zooxanthellae. Results suggest that space availability and the size of the algal symbionts determines the algal densities in the coral colonies. The large increases in the algal densities reported in corals exposed to elevated nutrient concentrations (i.e between a two- and five-fold increase in the algal standing stock) are not consistent with this theory. We suggest that increases of this magnitude are a product of the experimental conditions: reasons for this statement are discussed. We propose that the stability of the coral–algal symbiosis under non-stress conditions, and the constancy of zooxanthellae densities in corals reported across growth form, depth and geographic range, are related to space availability limiting algal densities. However, at these densities, zooxanthellae have attributes consistent with nutrient limitation.